Flat light sources that are used for back-lighting in devices such as liquid crystal displays (LCDs) are known in the art, as described below.
A first known type of flat light source is an edge-lit system that uses a flat optically transmissive plate as an optically conductive member. The flat light source used in this system causes light to be incident on one or both side edge surfaces of the optically conductive member which is formed of a transparent flat plate. Total reflection within the flat optically transmissive plate is utilized to propagate the light throughout the entire optically conductive plate. Part of the thus propagated light becomes diffused reflected light of less than the critical angle from a light-scattering reflective plate on the rear surface of the optically conductive plate, and thus diffused light is emitted from the outer surface of the optically conductive plate (refer to Japanese Utility Model Laid Open No. 55-162,201).
A second known type of flat light source has a lens sheet wherein one surface has projections and the other surface is smooth, which is placed with the projection side thereof on the outer surface of the optically conductive plate of the flat light source of the above first type. The light-focusing action of this lens is utilized to ensure that the diffused, reflected light is diffused uniformly and isotropically within a predetermined angular range (refer to Japanese Utility Model Laid Open No. 4-107,201).
The above described lens sheet could be used in combination with a frosted transparent diffusion plate (a frosted transparent sheet) formed by dispersing particles of a light-diffusing agent such as TiO.sub.2 within a transparent plastic. In such a case, the optical energy of the light source is distributed in a more concentrated manner within a predetermined limited angular range, than when a frosted transparent diffusion plate alone is placed over the optically conductive plate (refer to U.S. Pat. No. 4,729,067 and Japanese Patent Laid Open No. 61-55,684). Moreover, a uniform and highly isotropic diffused light can be obtained within this angular range.
However, both of the above prior-art techniques have problems. The first one simply places a light-scattering reflective plate on the rear surface of the optically conductive plate so that the emitted light has a comparatively sharp distribution that peaks at an angle of 60 degrees to the normal of the surface of the optically conductive plate. Therefore a phenomenon is observed in which the degree of luminance is insufficient in the normal direction (the forward direction) where brightness is most required, while optical energy is wasted in the lateral directions where it is completely unnecessary.
The second prior-art technique has a problem in that, when a lenticular sheet that comprises an array of a large number of individual triangular prismatic lenses is superimposed on the light-emitting surface of the optically conductive plate as the lens sheet, the ratio of optical energy emitted within angles between 30 and 60 degrees from the normal to the light-emitting surface is comparatively high, but even if the portion within 2 to 4 cm of the side edge portion of the optically conductive plate is very bright, the luminance drops gradually further away from this portion so that the edge at the opposite side from the light source is noticeably darker.
If a frosted transparent scattering diffusion plate is used, a further problem arises in that the particles of light-diffusing agent within the optically conductive plate absorb some of the light, so that the optical energy thereof is lost.
In addition, interference patterns such as Newton's rings could be generated by the optical seal between the lens sheet and the surface of the optically conductive plate.
Techniques that have been tried to solve these problems include:
1. An attempt to correct and make uniform the luminance distribution within the surface of the optically conductive plate by creating a pattern in a light-scattering reflective layer on the rear surface of the optically conductive plate, such as a dot pattern, in such a manner that the surface area of the pattern is decreased closer to the light source and increased further away therefrom, as disclosed in Japanese Patent Laid-Open No. 1-245,220 and Japanese Utility Model Laid-Open No. 6-15,008.
2. An attempt to correct and make uniform the luminance distribution within the surface of the optically conductive plate by disposing two or more light sources at the side edge portions of the optically conductive plate, as disclosed in Japanese Patent Laid-Open No. 3-9306.
3. An attempt to obtain a directed output light that has a substantially uniform luminance from the entire surface of the optically conductive plate, by providing a linear prismatic array (an array of prismatic lenses) that partially reflects and partially passes light on either the front or rear surface of the optically conductive plate, and varying the angle of inclination of the surfaces of these prisms and locally varying the thickness of the optically conductive plate, as disclosed in Japanese Patent Laid-Open No. 62-3226.
All of the above measures, and others, have problems in that it is difficult to provide a completely uniform luminance thereby. In addition, technique 1 has a further problem in that the dot pattern of the light-scattering reflective layer is visible from the side from which light is emitted. Technique 2 has a further problem in that the space required for the entire light source and the power consumption thereof are more than doubled.
Technique 3 has problems in that the form of the optically conductive plate is complicated, the fabrication of this design is extremely difficult, and it is also difficult to make the dot pattern of the light-scattering reflective layer invisible.
An objective of the present invention is therefore to solve the above problems with the prior art and provide a flat light source that implements a uniform and very bright light that is limited to a predetermined angular range, and that has no variations in luminance due to position within the light surface, without increasing the power consumption, amount of heat generated, or the size of the entire apparatus.